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Australian Intermediate Mathematics Olympiad 2020 Question 9

AIMO AUSTRALIA · Australian Intermediate Mathematics Olympiad 2020

Question 9

If $k,m,n$ are integers such that $2\le k\le m\lt n$, show that
$$\frac{m!}{m^k(m-k)!}\lt \frac{n!}{n^k(n-k)!}.$$
Note that $0!=1$ and $r!=1\times2\times3\times\cdots\times r$ for any positive integer $r$.